Macrophage-like U-937 cells were exposed in vitro to noncoherent 660 nm wavelength light produced by a superluminous diode at a power density of 120 mW/cm² over a range of energy densities from 2.4-9.6 j/cm². Twelve hours after irradiation, the medium in which the macrophages. had been growing and which contained growth factors secreted by them was added to 3T3 fibroblasts, also growing in in vitro. and its, effect on fibroblast proliferation assessed It was found that by 96 h after addition of the macrophage-conditioned medium there was a statistically significant increase in fibroblast proliferation in the cells exposed lo supernatants from sham-irradlatet1 macrophages. The most effective energy density was found to be 7.2J/cm² and the least effective 9.6 J/cm².